Single-acting and double-acting steam engines have provided power for industry and other uses for a long period of time. The single-acting steam engine may resemble a two and a four-stroke internal combustion engines in that a piston, connecting rod and crank are used per cylinder set. With the double-acting form of steam engine, straight line reciprocating motion is described not only by each piston, but also by each piston rod and crosshead. Motion is transferred from the crosshead via a connecting rod to the crank. The piston rod passes through a seal in the end of the cylinder and the steam is valved to work on the piston from above and also below it. This gives a “one stroke” action. With two double-acting cylinders, only four valves are required on a “full” uniflow engine of conventional design as against sixteen valves being required for an eight-cylinder four-stroke engine which exerts the same number of power impulses per revolution.
The uniflow engine exhaust system uses holes in the cylinder which are exposed to the top end of the cylinder adjacent to the piston near the bottom of its stroke. The same row of holes are exposed to the bottom or crank end of the cylinder adjacent to the piston near the top of its stroke. The length of the piston adjacent to the cylinder wall is equal or approximately equal to the stroke minus the diameter or length of the exhaust holes. (The exhaust holes in the cylinder can be seen in one of the photos on display). Clearance volume is provided at each end of the cylinder to allow for reasonable compression to take place at each end of a stroke.
A semi-uniflow engine is one in which exhaust valves are used to supplement the action of the exhaust holes in the cylinder wall. By employing the exhaust valves, the point at which compression begins on the return stroke of the piston can be delayed. Such an auxiliary exhaust feature is useful especially where exhaust is at atmospheric pressure rather than into a vacuum and/or, further, where compounding is utilized. Further, in single cylinder engines which are not necessarily self-starting, the auxiliary exhaust makes the engine easier to start. This is because it is easier before the admission steam starts the engine to rotate the engine against compression since with an auxiliary exhaust system compression acting against the piston begins later on the compression stroke.
In some early uniflow engines with auxiliary exhaust systems, the auxiliary or secondary exhaust steam traveled out through the same ports and passages through which previously admission steam entered. A disadvantage of this design is that the cooling effect of the exhausting steam lowered the efficiency of the engine. In other early semi-uniflow engines the auxiliary or secondary exhaust steam exhausted through special ports in the cylinder wall at positions between the main uniflow exhaust and the admission passages, the latter located near the cylinder ends.
Special valves such as poppet valves controlled these auxiliary exhaust passageways. These engines, if of the double-acting type, were fitted with four valves: two for inlet steam—one at each end of the cylinder, and two for auxiliary exhaust—one for the upper part of the cylinder and one for the lower part of the cylinder. A disadvantage of this design with its four valves plus the respective valve motions required for their operation is relative complexity. [See Skinner. P271. “Power from Steam,” R. L. Hills.]